External fixation frames may be used to correct skeletal deformities. The Ilizarov external fixation devices, for example, are widely used for this purpose. The Ilizarov-type devices may be used to translate bone segments by manipulating rings connected to each bone segment and a plurality of threaded rods connected to the manipulation rings.
A bone fragment can be moved, in general, from its original position as in a nonunion or malunion or from its intended position as in congenital deformities along six separate axes, a combination of three orthogonal translational axes (e.g., typical “X,” “Y” and “Z” axes) and three orthogonal rotational axes (e.g., rotation about such typical “X,” “Y” and “Z” axes). This movement generally occurs via manipulation of one or more adjustable length struts connected at each end thereof to rings of the external fixation device.
External fixation devices are generally attached to the boney skeleton, such as the femur or tibia, for example, with threaded and/or smooth pins and/or threaded and/or smooth and/or beaded wires. Such constructs are commonly referred to as orthopedic external fixators or external skeletal fixators. External fixators may be utilized to treat acute fractures of the skeleton, soft tissue injuries, delayed union of the skeleton when bones are slow to heal, nonunion of the skeleton when bones have not healed, malunion whereby broken or fractures bones have healed in a malposition, congenital deformities whereby bones develop a malposition, and bone lengthening, widening, or twisting.
Examples of external fixation devices include those described in U.S. Pat. No. 8,333,766 and U.S. patent application Ser. No. 13/592,832, the entire disclosures of both of which are hereby incorporated by reference herein. After a bone is fixed to such an external fixation device, one bone fragment may be moved in any combination of the six degrees of freedom relative to a fixed bone fragment. The movement is generally directed by a preoperative and/or postoperative plan defining precise movement of rings and/or struts to result in precise movement of one bone fragment relative to a fixed bone fragment. Despite following instructions provided by the plan, the deformity correction may not occur according to plan for a variety of reasons. Such errors between planned movements of the bone fragment relative to the fixed bone fragment may occur, for example, because of incorrect adjustments to the external fixation device or bending and other deformation of wires, pins, or other devices connecting the bone fragments to the external fixation device. Errors may also result from imprecision in, or deformation of, other components of the external fixation device, or from human error in following the deformity correction plan.
One way of determining whether planned movements are occurring correctly are markings on the struts themselves which may include numbers on an outside body of a strut indicating the length of the strut measured between ends thereof. As a particular strut is compressed or expanded, the change in length of the strut can be visualized via the markings. Such marking are found on adjustable length struts as shown, for example, in U.S. Pat. No. 6,030,386. In other devices, the change in length of struts is digitally displayed on the struts themselves such as shown in U.S. Pat. Pub. No. 2002/0010465. However, not only is it difficult to correlate the change in length of a strut and the position and orientation of respective bone fragments, this relationship is not always accurate based on the aforementioned deformation of wires, pins, or other devices connecting the bone fragments to the external fixation device. There is need for a device or method for tracking the movements of bone fragment relative to one another to determine if such movements are occurring accurately according to a deformity correction plan.